devops(observability): fix Prometheus port binding, scrape port, and update DEPLOYMENT.md

- Fix spring-boot scrape target from backend:8080 to backend:8081 (actuator/management port)
- Restrict Prometheus host port binding to 127.0.0.1 to prevent unintended external exposure
- Add observability stack (Prometheus, Node Exporter, cAdvisor) to topology description
- Add PORT_PROMETHEUS env var to DEPLOYMENT.md reference table

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.observability.yml

@@ -24,7 +24,7 @@ services:
       - '--storage.tsdb.retention.time=30d'
       - '--web.enable-lifecycle'
     ports:
-      - "${PORT_PROMETHEUS:-9090}:9090"
+      - "127.0.0.1:${PORT_PROMETHEUS:-9090}:9090"
     healthcheck:
       test: ["CMD", "wget", "-qO-", "http://localhost:9090/-/healthy"]
       interval: 30s